1. Field of the Invention
The invention relates to a filter apparatus suitable for use, for example, as a low pass filter of a receiving apparatus of a direct conversion system.
2. Description of the Related Arts
Hitherto, a receiving circuit of a radio communicating apparatus such as a cellular phone generally has had a construction of a super heterodyne system such that a received radio frequency (RF) signal is down-converted into an intermediate frequency (IF) signal and, thereafter, a base band signal is demodulated. FIG. 1 shows an example of the receiving circuit of such a super heterodyne system.
In FIG. 1, an RF signal received by an antenna 101 is amplified by an LNA (Low Noise Amplifier) 102 and, thereafter, supplied to a multiplier 104. A local oscillation signal is supplied to the multiplier 104 from a PLL (Phase Locked Loop) synthesizer 103. The RF signal and the local oscillation signal from the PLL synthesizer 103 are multiplied by the multiplier 104 and the reception RF signal is down-converted into an IF signal.
An output of the multiplier 104 is supplied to a band pass filter (BPF) 105. For example, an SAW (Surface Acoustic Wave) filter is used as a band pass filter 105. An output of the band pass filter 105 is supplied to multipliers 107A and 107B.
An output of a local oscillator 108 is supplied to the multiplier 107A. The output of the local oscillator 108 is phase-shifted by 90xc2x0 through a phase shifter 109 and supplied to the multiplier 107B.
The IF signal and the output of the local oscillator 108 are multiplied by the multiplier 107A. The IF signal and the output of the local oscillator 108 which was phase-shifted by 90xc2x0 by the phase shifter 109 are multiplied by the multiplier 107B. An I signal is demodulated by the multiplier 107A. A Q signal is demodulated by the multiplier 107B. Outputs of the multipliers 107A and 107B are supplied to an orthogonal demodulating circuit 110.
As mentioned above, hitherto, the receiving circuit of the radio communicating apparatus has used the super heterodyne system such that after the received RF signal is down-converted into the IF signal, the base band signal is demodulated.
According to the super heterodyne system, however, since an image frequency is generated when the RF signal is down-converted into the IF signal, it is necessary to provide a band pass filter for an IF circuit. A SAW filter is used as such a band pass filter. It is, therefore, difficult to form an integrated circuit and it becomes an obstacle to miniaturization.
Therefore, as such a receiving circuit of a mobile wireless apparatus, a method of reducing a circuit scale by using a direct conversion system is considered. According to the direct conversion system, since no image frequency is generated, the band pass filter for removing the image frequency can be omitted.
In the case of using the direct conversion system, a low pass filter is inserted after the I signal and Q signal are orthogonally demodulated by the multiplier. As such a low pass filter, since the base band signal is directly demodulated, a frequency which is handled is difficult to use the SAW filter.
Therefore, an active filter comprising a semiconductor device, a resistor, and a capacitor is generally used as such a low pass filter.
However, the active filter has a problem such that characteristics are changed due to a fluctuation in power source voltage or variations on processes and temperature characteristics of a transistor, the resistor, and the capacitor. Particularly, it is demanded to make cut-off characteristics constant in order to realize stable attenuating characteristics.
It is, therefore, an object of the invention to provide a filter apparatus and a filter control method in which characteristics can be made constant irrespective of a fluctuation in power source voltage or variations on processes and temperature characteristics and to provide a receiving apparatus using such a filter apparatus.
According to a preferred aspect of the invention, there is provided a filter apparatus comprising: a main filter which has an input terminal and an output terminal and whose characteristics can be set by an external control signal; a subordinate filter having substantially the same construction as that of the main filter; a signal generator for generating a signal of a frequency equal to a cut-off frequency of each of the main filter and the subordinate filter and supplying it to the subordinate filter; a phase difference detector for detecting a phase difference between the signal generated from the signal generator and an output signal of the subordinate filter and outputting a phase difference signal; a reference signal generator for generating a reference signal corresponding to an ideal value of the phase difference detected by the phase difference detector; and an error detector for detecting an error between the phase difference signal and the reference signal and supplying an error signal as an external control signal to the main filter and the subordinate filter.
A phase shift amount which is caused in a subordinate low pass filter at the time of the cut-off frequency is detected. A detection value of the phase shift amount which is caused in the subordinate low pass filter at the time of the cut-off frequency is compared with a reference value at the time of the cut-off frequency which is obtained from phase characteristics. By controlling the cut-off frequency of the subordinate low pass filter on the basis of an output of the comparison, the cut-off frequency of the subordinate low pass filter is controlled to a desired frequency and the cut-off frequency of the main low pass filter can be controlled to a desired frequency.
A comparison output between the detection value of the phase shift amount which is caused in the subordinate low pass filter at the time of the cut-off frequency and the reference value at the time of the cut-off frequency which is obtained from the phase characteristics is obtained by a multiplier. The reference value is generated by a multiplier having characteristics similar to those of such a multiplier. Thus, the errors and variations on the circuits, temperature characteristics, and the like can be cancelled.
The above and other objects and features of the present invention will become apparent from the following detailed description and the appended claims with reference to the accompanying drawings.